Re: [boost] Safe Float design question

One more thing. Is there a need to keep every single aspect separate? E.g., is there a point in requiring that overflow is checked, but underflow is ignored? I see the point in in treating inaccurate representation as a separate thing (because you will not want to have it most of the time), but things like overflow and underflow -- do you not want them to always go in pair? Also, given that safe_float<double> is to be a "drop-in" replacement for double, type double is convertible to types int, unsigned int and bool. Is safe_float<double> also going to be convertible to these types? If the answer should be yes, do these conversions also require a dedicated policy? Regards, &rzej; 2018-07-17 10:55 GMT+02:00 Andrzej Krzemienski :

Hi Damian, I may not be answering your question directly, but maybe the following will help.

You mention checking things as: - I would like to check there was no "overflow_to_infinite" in "addition operations", otherwise I "throw exception".. - I would like to check there was no "division by zero" in "division", otherwise I will "log to cerr and ignore it".

The two checks are very different in nature, and I do not even think they belong into the same library. The first check traps a situation that (1) the programmer-user cannot control (you do not know in advance if the operation would overflow, it is not a bug to cause overflow), (2) Type double has an inconvenient way of handling the situation (does not report an error), so you want your type to handle it better.

The later check checks for programmer mistakes. These mistakes should not be handled at run-time, but corrected in the code (the overflow cannot be easily corrected in the code), and it adds overhead for situations that do not occur for correct programs. The precondition violation should be handled by contract libraries: otherwise you are preventing static-analysis tools and UB sanitizers from finding division-by-zero bugs.

Next, if you go with providing checks separately from the method of reporting them:

using sf = safe_float;

You will loose the ability to report different kinds of errors in different ways. You have to make the call: do you always want all checks to be reported in the same way, or do you want, for instance to allow reporting the inaccurate addition in a different way than overflowing to infinity? If the latter, you will have to encode the method of handling inside the check:

using P = compose_policy< check_addition_overflow< report_throw_on_failure >, check_division_underflow< abort_on_failure >

...

;

The design, where I first need to write 10 lines to build a policy, and then define a type:

using fp = safe_float;

seems the right choice to me. Of course, you should provide some predefined policies, that would be most obviously chosen most of the time. (Like, check and throw only underflow and overflow, check and abort only on underflow and overflow).

Regards, &rzej;

Finally 2018-07-17 4:18 GMT+02:00 Damian Vicino via Boost :

...

Hi, I looking for some design advice.

The context: - I'm revisiting SafeFloat after some time and, while doing so, I'm rethinking all the decisions from the past. Idea is to sent for review in the following months. - The goal of the library is being a drop-in replacement for float, that adds checks to floating point operations and reports when a check failed.

Without SafeFloat, someone could write something like this:

#include <iostream> #include <limits> #include <cfenv>

using namespace std;

int main(){ float a = 1.0f; float b = numeric_limits<float>::max(); feclearexcept(FE_ALL_EXCEPT); float c = a/b; if(fetestexcept(FE_UNDERFLOW)) { cout << "underflow result\n"; } }

What I expect when using safe_float is to declare upfront "what checks" I care about, "what operations" to check, and "what to do when a check fails".

Some intention examples: - I would like to check there was no "overflow_to_infinite" in "addition operations", otherwise I "throw exception".. - I would like to check there was no "division by zero" in "division", otherwise I will "log to cerr and ignore it". - I would like to check there was no "inexact" "addition", otherwise I return an boost::unexpected.

I have at least 5 things to check (there is 5 flags in c++11::fenv). I have at least 4 places to check (+/-/*//) and I want to keep what to do about it customizable.

In addition, sometimes I want to check multiple things "overflow and underflow", etc...

So, the question is how the user can pass all that information to the type and it doesn't look as horrible nonsense.

My original option was: int main(){ using CHECK = compose_policy::type; using REPORT = report_throw_on_failure; using sf = safe_float;

try{ sf a = 1.0_sf; sf b = numeric_limits<sf>::max(); auto c = a/b; } catch (safe_float_exception e){ cout << e.message(); //this outputs there was a underflow }

}

Please comment in what you think about this way to use. Is there a better way to specify the policies to apply that I should try?

Best regards, Damian

I expect when using safe float to write some code like this:

int main(){

safe_float<float>

}

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